Kind-of-crack AU idea which doesn't actually have to be crack:

Sherlock is a professional home-wrecker. When rich men want to get out of unwanted marriages, they hire Sherlock to "seduce" their wives (with or without actual sex) and then they can accuse their wives of infidelity in the divorce proceedings. With Sherlock's good looks and incredible ability to deduce exactly what women want, he's got no difficulty taking on interesting "cases" and the ethics don't bother him a bit.

Lestrade's wife presents an interesting gender twist: she hires Sherlock to seduce Lestrade, in an attempt to draw attention away from her own infidelities and come out ahead in the divorce. Only problem is, even though Lestrade and Sherlock are both strangely happy to make googly eyes at each other, Lestrade is unwilling to cheat on his wife. And Sherlock is finding himself more and more attracted to the DI because of it.

Totally up to you where this goes (or how much of this plot bunny you follow), as long as it involves some hot Sherstrade sex and a happy ending :-)